This invention relates generally to electronic systems. More particularly, this invention relates to a technique for utilizing a lossy dielectric substrate in a high speed electronic system.
A dielectric or electrical insulator is a material with substantially limited electrical conductivity. In particular, typical electrically insulating materials have a volume resistivity of 1012 ohm*m. A large number of commercial electrical insulators fall into, but are not limited to, two broad classes: ceramics and polymers (including thermoplastics and thermoset plastics). Widely used dielectric materials include vacuum, paper impregnated with oil or wax, polyester, polystyrene, polyimide, epoxy, fluorocarbon plastic, mica, glass, alumina ceramic, and glass ceramic.
Dielectric behavior in materials is often defined in terms of a parallel-plate electrical capacitor. When an electric field is established between the capacitor plates or electrodes, an electric charge forms on the plates. The electric field produces a charge density on the plates proportional to the field. For an evacuated gap between the plates, the constant of proportionality is called the permittivity of free space (∈0) If the vacuum between the plates is replaced by a dielectric material, the charge density will increase for the same value of applied electric field; the permittivity (∈) of the dielectric material is greater than that of free space. The charge density increase comes from the polarization of the dielectric in the presence of the applied electric field. One can form a ratio of ∈ and ∈0 to get a dimensionless number called the dielectric constant (K). The dielectric constant K equals 1 for vacuum.
Low values for K are desired for high speed electronics systems. Typical K values for dielectric substrates, based on glass-polymer resin systems, range from 2.5 to 5.0. Commercial materials include glass-reinforced epoxy and polyimide thermoset plastics and fluorocarbon plastic-based laminates. In general, the closer K is to unity, the more costly the substrate material.
Another important property of dielectrics for electronic use is dielectric loss. Dielectric loss is the measure of transformation of electrical energy into heat and is generally considered not desirable. The electric field-induced polarization occurring in the dielectric material is reversible in a time-varying electric field. However, the polarization reversal takes finite time and energy loss occurs as a function of the frequency of the electric field variation. For the capacitor with a lossy dielectric, the effect is that the plate charge density is out of phase with the applied variable electric field. Stated another way for the capacitor example, the current leads the voltage by exactly 90xc2x0 for the perfect dielectric material. In an actual dielectric, the current leads the voltage by 90xc2x0-xcex4, where the angle xcex4 is proportional to the dielectric energy loss. To keep xcex4 (expressed as xe2x80x9closs factorxe2x80x9d or K*tan xcex4 in practice) small, system designers commonly use expensive dielectric materials. For example, E-glass (borosilicate electrical glass) is used in place of ordinary soda-lime glass for laminated circuit dielectric applications. Expensive fluorocarbon polymers are often used in microwave circuit applications.
FIG. 1 illustrates a prior art system including a signal generator 20, which generates a signal 22. The signal 22 transitions from a digital low value to a digital high value relatively quickly and therefore is referred to as having a relatively fast edge rate. The signal 22 is applied to a transmission system with a standard dielectric 24. The transmission system includes a conductor positioned on a standard dielectric. The transmission system 24 causes the edge rate of the original signal 22 to be delayed slightly, as shown with signal 26.
Expensive dielectric materials are used in prior art transmission systems of the type shown in FIG. 1 so that the edge rate of the originally generated signal is minimally altered. Although most prior art systems endeavor to minimize signal transition delay through the use of expensive dielectrics, there is a limited class of high speed electronic systems in which energy dissipation is desired. One way of dissipating energy in such systems is to add discrete energy dissipating components, such as resistive elements, e.g., discrete resistors. Unfortunately, this approach increases system cost.
In view of the foregoing, it would be highly desirable to identify digital systems in which it is not necessary to utilize expensive, low loss dielectric materials. It would also be desirable to identify a low cost lossy dielectric substrate for use in such systems.
The invention includes an electronic device. The electronic device has a standard dielectric material and a lossy material integrated with the standard dielectric material to selectively control the distributed resistance of the standard dielectric material. The lossy material may be integrated with the standard dielectric material through insertion. The inserted material may be resistive particles, carbon particles, open cell conductive foam, carbon impregnated open cell conductive foam, and the like. The lossy material may be integrated with the standard dielectric material by attaching a loss inducing physical structure to the standard dielectric material. The loss inducing physical structure may be a planar resistive layer attached to the standard dielectric material. The planar resistive layer may have an extended surface to attenuate high frequency signals.
The invention also includes a method of constructing an electronic device. The method includes the step of providing a standard dielectric material. The standard dielectric material is integrated with a lossy material so as to selectively control the distributed resistance of the standard dielectric material.
The invention achieves frequency response shaping through predetermined combinations of a standard dielectric material and a lossy material. The invention""s incorporation of a lossy material with a standard dielectric material results in a low cost substrate that improves performance in selected high speed digital systems.